How can I customize the background to the body?

It can be done with the custom css. In your Dashboard: Apparence -> Customize -> Custom CSS

Use this code for a background image

body {
background-image: url('url-image');
background-repeat: no-repeat;
background-size: cover; 
background-attachment: fixed;
}

Use this code for a background pattern

body {
background-image: url('url-image');
background-repeat: repeat;
}